4-2
Figure 4-1A.—Block diagram of a basic power supply.
As illustrated in view B of figure 4-1, the first section is the TRANSFORMER. The transformer
steps up or steps down the input line voltage and isolates the power supply from the power line. The
RECTIFIER section converts the alternating current input signal to a pulsating direct current. However, as
you proceed in this chapter you will learn that pulsating dc is not desirable. For this reason a FILTER
section is used to convert pulsating dc to a purer, more desirable form of dc voltage.
Figure 4-1B.—Block diagram of a basic power supply.
The final section, the REGULATOR, does just what the name implies. It maintains the output of the
power supply at a constant level in spite of large changes in load current or input line voltages.
Now that you know what each section does, let's trace an ac signal through the power supply. At this
point you need to see how this signal is altered within each section of the power supply. Later on in the
chapter you will see how these changes take place. In view B of figure 4-1, an input signal of 115 volts ac
is applied to the primary of the transformer. The transformer is a step-up transformer with a turns ratio of
1:3. You can calculate the output for this transformer by multiplying the input voltage by the ratio of turns
in the primary to the ratio of turns in the secondary; therefore, 115 volts ac
3 = 345 volts ac (peak-to-
peak) at the output. Because each diode in the rectifier section conducts for 180 degrees of the 360-degree
input, the output of the rectifier will be one-half, or approximately 173 volts of pulsating dc. The filter
section, a network of resistors, capacitors, or inductors, controls the rise and fall time of the varying
signal; consequently, the signal remains at a more constant dc level. You will see the filter process more
clearly in the discussion of the actual filter circuits. The output of the filter is a signal of 110 volts dc,
with ac ripple riding on the dc. The reason for the lower voltage (average voltage) will be explained later
in this chapter. The regulator maintains its output at a constant 110-volt dc level, which is used by the
electronic equipment (more commonly called the load).
Q1. What are the four basic sections of a power supply?
Q2. What is the purpose of the rectifier section?
